The potential motion of a hoisted object can best be envisioned by a Cartesian coordinate system in which the z-axis is in the vertical direction, and the x and y axes form the horizontal plane. The rotation of the hoisted object about the z-axis is therefore defined as yaw, rotation about the x-axis is defined as pitch, and rotation about the y-axis is defined as roll.
In typical load transporting applications, a crane will have a single lifting cable, which is stable only in the z-direction. If an external force is applied from the sides, the load will either roll, pitch, or yaw, or will sway in the x- and y-directions.
While the loads are being hoisted, it is essential that the center of mass of the assembly formed by the hoisting apparatus and the load, is vertically aligned with the hoisting point in order to have the assembly balanced. Otherwise, the assembly may rock and swing, causing damages to the part itself, to the surrounding equipment or even causing injuries to human operators.
Therefore, the prior art has long recognized the need to compensate for these undesired motions, and as a result various solutions have been developed for stabilizing a hoisted load. For example, U.S. Pat. No. 4,883,184 describes a cable arrangement and lifting platform in a stabilized manner. The lifting platform secures loads to a securing device and the platform is able to be suspended from a crane by an attachment carriage. The attachment carriage includes a cable winch onto which six cables suspend and attach to the lifting platform. The attachment carriage also includes cable guides which guide the six cables away from the winch in three cable pairs, preferably equidistantly-spaced.
In order to secure the cables to the lifting platform, the platform includes an attachment frame having three cable attachment points, preferably spaced equidistantly apart with respect to each other. The lifting platform helps stabilize the lifting of loads by sensing the load's imbalance relative to the center of mass of the platform and repositioning the load to correct for the imbalance.
The U.S. Pat. No. 4,932,541 describes a stabilized cargo-handling system using means for stabilizing a suspended cargo in all six degrees of freedom using six individually controlled cables in tension in a kinematic arrangement. Inertial and distance sensors, coupled with cable drives, control the multi-cabled crane automatically.
On the other hand, six degrees of freedom actuation devices, generally known as hexapods, are commonly used for example in flight or driving simulators, which are capable of moving a platform on which a simulation cabin is mounted, with six degrees of freedom in space. The best known prior art mobile platform, is a Stewart platform, which is based on the use of a hexapod positioning device allowing motion with six degrees of freedom. The type of motion of these platforms forms part of the family of parallel robots.
The U.S. Patent Publication Nos. 2009/0035739A1 and 2012/0180593A1 describe and illustrate in more detail examples of Stewart platforms Typically, a Stewart platform comprises a fixed lower plate, six telescopic actuators and a mobile upper plate, wherein the telescopic actuators are pivotally connected at their opposite ends to the base plate and to the mobile upper plate, there being three attachment points on each of the base plate and mobile upper plate to which respective pairs of the telescopic actuators are connected. As a consequence of this known arrangement, the mobile upper plate has six degrees of freedom, that is, both rotation and translation about the X-, Y- and Z-axes.
Cable-suspended robots or tendon-driven robots, generally referred as cable robots, are also known, and are based on multiple cables attached to a mobile platform that may carry one or more manipulators or robots. The platform is manipulated by motors that can extend or retract the cables. Cable robots are used for various manipulation tasks in a three-dimensional workspace, as for example material handling, haptics, etc. The U.S. Patent Publication No. 2009/0066100A1 refers to a cable robot of this type.
In the aeronautical industry, large and heavy parts like horizontal tail planes, wings or fuselage sections, have to be hoisted and transported from one working station to another within a factory or assembly plant. For this task, hosting mechanisms, such as overhead cranes or winches are commonly used to provide the necessary lifting force to lift the part.
Hosting and positioning these large aircraft parts is a challenge because a large variety of parts of different sizes and weights, of previously unknown position of the center of gravity, have to be transported and handled within a factory. A classical solution, is to provide a dedicated lifting equipment for each part, but this solution is very expensive and cumbersome, since a large number of hosting equipment (jigs) are required.
Consequently, although many self-balanced load hoisting systems are already known, none of them has been specifically conceived for solving the problems of hoisting and handling large aircraft parts in the aeronautical industry